DolphinWX: Fix the video dialog crashing Dolphin on OSX

When a game was running and someone opened the video dialog, it would crash. This is because the preprocessor macro should have been __APPLE__ not _APPLE_

Fixes issue 7644.
Merge pull request #1139 from lioncash/videodlg-osx

DolphinWX: Fix the video dialog crashing Dolphin on OSX